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Botulinum toxin type A, a potent neuromuscular block-ing agent, has been used for a variety of neuromuscu-lar disorders, including cervical dystonia and ble-pharospasm.1 Recently, this drug has been used in the management of chronic lateral epicondylitis (tennis el bow).2–5 The extensor muscles of the wrist are paralyzed with the aim of releasing tension on the common extensor tendon and caus-ing gradual healing of the epicondylitis. This approach is based on the same rationale as surgical release of the extensor carpi radialis brevis tendon.3 The most important factor in successfully paralyzing any muscle is the site of injection of the botulinum toxin.1 Injection at a fixed distance from anatomic landmarks, as was per-formed in previous clinical trials of botulinum toxin for the management of lateral epicondylitis,2–5 would result in inade-quate paralysis of the intended muscle. It has been suggested that the extensor digitorum muscle, which is innervated by deep branches of the radial nerve, plays an important role in the development of lateral epicondylitis.6 We therefore chose to determine the injection site based on each patient’s forearm length and the course of the posterior interosseus nerve to achieve adequate paralysis of the extensor digitorum muscle. We conducted a randomized controlled trial to test whether this approach would be effective in reducing pain and improv-ing grip strength. Methods Study design The study was performed at the Imam Khomeini Hospital Complex, a 1230-bed tertiary care referral hospital affiliated with the Tehran University of Medical Sciences that serves patients from all over Iran. The study protocol was approved by the hospital’s ethical committee. The participants of the study were chosen according to our inclusion and exclusion criteria (Box 1) from patients referred to our clinic from August 2007 to March 2008. All partici-pants had to have a history of failed previous therapeuti